The assembly used when compiling might be different than that used when loading and the type is missing.
Why the error? My WPF application UIXStandards has reference to a class library Controls which in turn has a reference to the Smt.Windows.Controls library. In Visual Studio when I add reference to Controls library from my UIXStandards application, Visual Studio does not copy any references needed for the Controls library (e.g . ) Smt.Windows.Controls to UIXStandards application. Solution: To solve the problem, within Visual Studio when you refer to a class library ( Controls ) also add references its dependencies ( Smt.Windows.Controls ).